1. Field of the Invention
The invention relates to a heat-sealable plastic film having a heat-sealable coating of a thermoplastic polystyrene resin.
2. Description of the Background
Plastic containers for the packaging of foodstuffs must in many cases be sealed in an air-tight manner in order to ensure their shelf lives. The seals used in many cases, in particular in the packaging of dairy products, are aluminum foils coated with a heat-sealable composition, which ensures adequate adhesion to a wide variety of plastic containers. Since aluminum is insensitive to heat, the heat-sealing operation can be carried out at any desired temperature. It need only be ensured that the seal is of sufficient quality that, on the one hand, the container is sufficiently tightly sealed, ie., for example, does not burst when dropped, but, on the other hand, can still easily be opened by hand without the foil tearing. It is important here that the seal satisfies this strength requirement over a certain processing range, as required by commercially available sealing equipment.
In addition to aluminum foils, plastic films with a heat-sealable coating are also used. The particular heat-sealable coating depends on the type of plastic from which the container to be sealed is produced. Thus, an acrylate resin-based coating with additives for regulating the adhesive strength and the melting point is applied, for example, to the hard PVC heat-sealable films most commonly used for this purpose.
In contrast to aluminum foils, heat-sealable plastic films can also be subjected to thermoforming. Shaped lids produced from plastic films can thus also be heat-sealed to the container to be sealed, but are suitable for re-sealing the package after removal of some of the contents.
However, the PVC heat-sealable films currently used have some disadvantages; they are relatively expensive, since they must be provided with a heat-sealable coating, and the residual solvent originating from the coating can only be removed from the plastic using considerable effort. In addition, environmental considerations increasingly mean that PVC is being replaced by other thermoplastics in disposable packages.
DE-A 3,531,036 describes plastic films which can be produced by coextrusion and have a heat-sealable coating comprising an impact resistant polystyrene, a block copolymer and a lubricant.
Films made from amorphous polyethylene terephthalate with a heat-sealable coating have also been disclosed. All these heat-sealable plastic films must be provided with a thin heat-stable protective coating on the side facing the sealer unit in order to prevent adhesion of the plastic film to the sealer jaws.
It has become apparent that a significantly narrower processing latitude is available for use of all heat-sealable plastic films compared with aluminum foils; if the sealing temperature and time are inadequate, the plastic film does not permit sufficient heat transmission, and adhesion of the film does not take place. However, if the sealing temperature and time are excessive, the plastic film melts and runs away under the pressure necessary for sealing. It then becomes so thin in the sealing area that it tears when only a small force is applied, and satisfactory opening and re-sealing of the package is no longer possible. Between these two extremes, there is, for all these heat-sealable plastic films, usually only a narrow range of from 10.degree. to 20.degree. C. which must be reliably observed in order to ensure trouble-free production and satisfactory use of the sealed package. Practical experience has shown that this is possible in a number of cases; however, particularly in filling plants in which several pots are sealed simultaneously, sufficiently reliable temperature control is not always ensured since temperature differences of 20.degree. C. and more are no exception between the various cavities.